The present invention relates to a method for catalytically reducing nitrogen oxides in exhaust gases containing nitrogen oxides, sulfur oxides and dusts which are produced by burning a heavy oil. More particularly, it relates to a method for the removal of nitrogen oxides from exhaust gases by a catalytic reduction process wherein a sealing gas is introduced into the portions of the moving catalyst bed reactor where the catalyst is taken out therefrom and is again supplied thereto, whereby the precipitation and adhesion of salts such as ammonium sulfate and ammonium hydrogen sulfate onto the catalyst are effectively prevented.
There have, recently, been developed various methods for the removal of nitrogen oxides from heavy oil combustion exhaust gases by catalytically reducing nitrogen oxides in the presence of ammonia.
It is known that a process for the removal of nitrogen oxides using a fixed bed type reduction reactor has various problems, for instance, it is very difficult to remove completely the dusts from a large amount of combustion exhaust gases and the dusts are built up onto the catalyst bed which results in increase of the pressure loss. Even if the dusts can be completely removed, it requires a high cost. Besides, the catalyst is poisoned by sulfur oxides and the activity of the catalyst is markedly decreased.
For the purpose of eliminating these defects, a moving catalyst bed has been proposed. That is, a part of the catalyst is taken out from the reactor, and the dusts accompanied with the catalyst are removed and optionally the catalyst is reactivated and then the catalyst thus treated is again supplied into the reactor.
However, according to this method, although the decrease of the catalyst activity due to poison by sulfur oxides may be prevented, the catalyst activity is still unfavorably decreased in case of a moving bed system. Besides, even if the catalyst poisoned by sulfur oxides is reactivated, the original activity of the catalyst can not sufficiently be recovered by such a reactivation treatment.
As the result of extensive studies by the present inventors, there have been found the causes of the decrease of catalyst activity and the method for preventing the decrease of catalyst activity. That is, it has now been found that in the process for catalytically reducing nitrogen oxides in the exhaust gases containing nitrogen oxides, sulfur oxides and dusts which is produced by burning a heavy oil using a moving catalyst bed reactor wherein the exhaust gases are contacted with ammonia in the presence of a catalyst of metal oxides (e.g. a catalyst produced by carrying oxides of vanadium, tungsten, molybdenum or the like onto a carrier such as alumina), the decrease of catalyst activity is induced by the adhesion of salts such as ammonium sulfate or ammonium hydrogen sulfate onto the catalyst at the portions of the moving catalyst bed reactor where the catalyst is taken out therefrom and is again supplied thereto, and further that the precipitation and adhesion of the salts can be prevented by introducing a sealing gas into the portions of the moving catalyst bed reactor where the catalyst is taken out therefrom and is again supplied thereto.
An object of the present invention is to provide a method for removing nitrogen oxides from exhaust gases containing nitrogen oxides, sulfur oxides and dusts by catalytical reduction thereof without a decrease of the activity of the catalyst.
Another object of the present invention is to provide a method for preventing the decrease of catalyst activity due to the poison thereof by sulfur oxides in a moving catalyst bed type process for the removal of nitrogen oxides.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and accompanying specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. These changes and modifications are intended to be encompassed within the scope of the invention.